The field of the present invention relates to high intensity blending apparatus, particularly for blending operations designed to cause additive materials to become affixed to the surface of base particles. More particularly, the proposed invention relates to an improved blending tool for producing surface modifications to electrophotographic and related toner particles.
State of the art electrophotographic imaging systems increasingly call for toner particles having narrow distributions of sizes in ranges less than 10 microns. Along with such narrow distributions and small sizes, such toners require increased surface additive coverage since increased quantities of surface additives improve charge control properties, decrease adhesion between toner particles, and decrease Hybrid Scavangeless Development (“HSD”) developer wire contamination in electrophotographic systems. The blending tool embodiments of the present invention enable a toner having a high degree of coverage by surface additives and having a high degree of adhesion of the surface additives to the toner particles. The present invention also relates to an improved method for producing surface modifications to electrophotographic and related toner particles. This method comprises using an improved blending tool to cause increased blending intensity during high speed blending processes.
A typical process for manufacture of electrophotographic, electrostatic or similar toners is demonstrated by the following description of a typical toner manufacturing process. For conventional toners, the process generally begins by melt-mixing the heated polymer resin with a colorant in an extruder, such as a Werner Pfleiderer ZSK-53 or WP-28 extruder, whereby the pigment is dispersed in the polymer. For example, the Werner Pfleiderer WP-28 extruder when equipped with a 15 horsepower motor is well-suited for melt-blending the resin, colorant, and additives. This extruder has a 28 mm barrel diameter and is considered semiworks-scale, running at peak throughputs of about 3 to 12 lbs./hour.
Toner colorants are particulate pigments or, alternatively, are dyes. Numerous colorants can be used in this process. A suitable toner resin is then mixed with the colorant by the downstream injection of the colorant dispersion. Examples of suitable toner resins which can be used include but are not limited to polyamides, epoxies, diolefins, polyesters, polyurethanes, vinyl resins and polymeric esterification products of a dicarboxylic acid and a diol comprising a diphenol.
Illustrative examples of suitable toner resins selected for the toner and developer compositions of the present invention include vinyl polymers such as styrene polymers, acrylonitrile polymers, vinyl ether polymers, acrylate and methacrylate polymers; epoxy polymers; diolefins; polyurethanes; polyamides and polyimides; polyesters such as the polymeric esterification products of a dicarboxylic acid and a diol comprising a diphenol, crosslinked polyesters; and the like. The polymer resins selected for the toner compositions of the present invention include homopolymers or copolymers of two or more monomers. Furthermore, the above-mentioned polymer resins may also be crosslinked.
Illustrative vinyl monomer units in the vinyl polymers include styrene, substituted styrenes such as methyl styrene, chlorostyrene, styrene acrylates and styrene methacrylates; vinyl esters like the esters of monocarboxylic acids including methyl acrylate, ethyl acrylate, n-butyl-acrylate, isobutyl acrylate, propyl acrylate, pentyl acrylate, dodecyl acrylate, n-octyl acrylate, 2-chloroethyl acrylate, phenyl acrylate, methylalphachloracrylate, methyl methacrylate, ethyl methacrylate, butyl methacrylate, propyl methacrylate, and pentyl methacrylate; styrene butadienes; vinyl chloride; acrylonitrile; acrylamide; alkyl vinyl ether and the like. Further examples include p-chlorostyrene vinyl naphthalene, unsaturated mono-olefins such as ethylene, propylene, butylene and isobutylene; vinyl halides such as vinyl chloride, vinyl bromide, vinyl fluoride, vinyl acetate, vinyl propionate, vinyl benzoate, and vinyl butyrate; acrylonitrile, methacrylonitrile, acrylamide, vinyl ethers, inclusive of vinyl methyl ether, vinyl isobutyl ether, and vinyl ethyl ether; vinyl ketones inclusive of vinyl methyl ketone, vinyl hexyl ketone and methyl isopropenyl ketone; vinylidene halides such as vinylidene chloride and vinylidene chlorofluoride; N-vinyl indole, N-vinyl pyrrolidone; and the like.
Illustrative examples of the dicarboxylic acid units in the polyester resins suitable for use in the toner compositions of the present invention include phthalic acid, terephthalic acid, isophthalic acid, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, maleic acid, fumaric acid, dimethyl glutaric acid, bromoadipic acids, dichloroglutaric acids, and the like; while illustrative examples of the diol units in the polyester resins include ethanediol, propanediols, butanediols, pentanediols, pinacol, cyclopentanediols, hydrobenzoin, bis(hydroxyphenyl)alkanes, dihydroxybiphenyl, substituted dihydroxybiphenyls, and the like. Resin binders for use in the present invention comprise polyester resins containing both linear portions and cross-linked portions of the type described in U.S. Pat. No. 5,227,460 (incorporated herein by reference above).
The resin or resins are generally present in the resin-toner mixture in an amount of from about 50 percent to about 100 percent by weight of the toner composition, and preferably from about 80 percent to about 100 percent by weight.
Additional “internal” components of the toner may be added to the resin prior to mixing the toner with the additive. Alternatively, these components may be added during extrusion. Various known suitable effective charge control additives can be incorporated into toner compositions, such as quaternary ammonium compounds and alkyl pyridinium compounds, including cetyl pyridinium halides and cetyl pyridinium tetrafluoroborates, as disclosed in U.S. Pat. No. 4,298,672, the disclosure of which is totally incorporated herein by reference, distearyl dimethyl ammonium methyl sulfate, and the like. The internal charge enhancing additives are usually present in the final toner composition in an amount of from about 0 percent by weight to about 20 percent by weight.
After the resin, colorants, and internal additives have been extruded, the resin mixture is reduced in size by any suitable method including those known in the art. Such reduction is aided by the brittleness of most toners that causes the resin to fracture when impacted. This allows rapid particle size reduction in pulverizers or attritors such as media mills, jet mills, hammer mills, or similar devices. An example of a suitable jet mill is an Alpine 800 AFG Fluidized Bed Opposed Jet Mill. Such a jet mill is capable of reducing typical toner particles to a size of about 4 microns to about 30 microns. For color toners, toner particle sizes may average within an even smaller range of 4–10 microns.
Inside the jet mill, a classification process sorts the particles according to size. Particles classified as too large are rejected by a classifier wheel and conveyed by air to the grinding zone inside the jet mill for further reduction. Particles within the accepted range are passed onto the next toner manufacturing process.
After reduction of particle size by grinding or pulverizing, a classification process sorts the particles according to size. Particles classified as too fine are removed from the product eligible particles. The fine particles have a significant impact on print quality and the concentration of these particles varies between products. The product eligible particles are collected separately and passed to the next toner manufacturing process.
After classification, the next typical process is a high speed blending process wherein surface additive particles are mixed with the classified toner particles within a high speed blender. These additives include but are not limited to stabilizers, waxes, flow agents, other toners and charge control additives. Specific additives suitable for use in toners include fumed silica, silicon derivatives, ferric oxide, hydroxy terminated polyethylenes, polyolefin waxes, including polyethylenes and polypropylenes, polymethylmethacrylate, zinc stearate, chromium oxide, aluminum oxide, titanium oxide, stearic acid, and polyvinylidene fluorides.
The amount of external additives is measured in terms of percentage by weight of the toner composition, and the additives themselves are not included when calculating the percentage composition of the toner. For example, a toner composition containing a resin, a colorant, and an external additive may comprise 80 percent by weight resin and 20 percent by weight colorant. The amount of external additive present is reported in terms of its percent by weight of the combined resin and colorant. The combination of smaller toner particle sizes required by some newer color toners and the increased size and coverage of additive particles for such color toners increases the need for high intensity blending.
The above additives are typically added to the pulverized toner particles in a high speed blender such as a Henschel Blender FM-10, 75 or 600 blender. The high intensity blending serves to break additive agglomerates into the appropriate nanometer size, evenly distribute the smallest possible additive particles within the toner batch, and attach the smaller additive particles to toner particles. Each of these processes occurs concurrently within the blender. Additive particles become attached to the surface of the pulverized toner particles during collisions between particles and between particles and the blending tool as it rotates. It is believed that such attachment between toner particles and surface additives occurs due to both mechanical impaction and electrostatic attractions. The amount of such attachments is proportional to the intensity level of blending which, in turn, is a function of both the speed and shape of the blending tool. The amount of time used for the blending process plus the intensity determines how much energy is applied during the blending process. For an efficient blending tool that avoids snow plowing and excessive vortices and low density regions, “intensity” can be effectively measured by reference to the power consumed by the blending motor per unit mass of blended toner (typically expressed as Watts/lb). Using a standard Henschel Blender tool to manufacture conventional toners, the blending times typically range from one (1) minute to twenty (20) minutes per typical batch of 1–500 kilograms. For certain more recent toners such as toners for Xerox Docucenter 265 and related multifunctional printers, blending speed and times are increased in order to assure that multiple layers of surface additives become attached to the toner particles. Additionally, for those toners that require a greater proportion of additive particles in excess of 25 nanometers, more blending speed and time is required to force the larger additives into the base resin particles.
The process of manufacturing toners is completed by a screening process to remove toner agglomerates and other large debris. Such screening operation may typically be performed using a Sweco Turbo screen set to 37 to 105 micron openings.
The above description of a process to manufacture an electrophotographic toner may be varied depending upon the requirements of particular toners. In particular, for full process color printing, colorants typically comprise yellow, cyan, magenta, and black colorants added to separate dispersions for each color toner. Colored toner typically comprises much smaller particle size than black toner, in the order of 4–10 microns. The smaller particle size makes the manufacturing of the toner more difficult with regard to material handling, classification and blending.
The above described process for making electrophotographic toners is well known in the art. More information concerning methods and apparatus for manufacture of toner are available in the following U.S. patents, each of the disclosures of which are incorporated herein: U.S. Pat. No. 4,338,380 issued to Erickson, et al; U.S. Pat. No. 4,298,672 issued to Chin; U.S. Pat. No. 3,944,493 issued to Jadwin; U.S. Pat. No. 4,007,293 issued to Mincer, et al; U.S. Pat. No. 4,054,465 issued to Ziobrowski; U.S. Pat. No. 4,079,014 issued to Burness, et al; U.S. Pat. No. 4,394,430 issued to Jadwin, et al; U.S. Pat. No. 4,433,040 issued to Niimura, et al; U.S. Pat. No. 4,845,003 issued to Kiriu, et al; U.S. Pat. No. 4,894,308 issued to Mahabadi et al.; U.S. Pat. No. 4,937,157 issued to Haack, et al; U.S. Pat. No. 4,937,439 issued to Chang et al.; U.S. Pat. No. 5,370,962 issued to Anderson, et al; U.S. Pat. No. 5,624,079 issued to Higuchi et al.; U.S. Pat. No. 5,716,751 issued to Bertrand et al.; U.S. Pat. No. 5,763,132 issued to Ott et al.; U.S. Pat. No. 5,874,034 issued to Proper et al.; and U.S. Pat. No. 5,998,079 issued to Tompson et al.
In addition to the above conventional process for manufacturing toners, other methods for making toners may also be used. In particular, emulsion/aggregation/coalescence processes (the “EA process”) for the preparation of toners are illustrated in a number of Xerox Corporation patents, the disclosures of each of which are totally incorporated herein by reference, such as U.S. Pat. Nos. 5,290,654, 5,278,020, 5,308,734, 5,370,963, 5,344,738, 5,403,693, 5,418,108, 5,364,729, and 5,346,797; and also of interest may be U.S. Pat. Nos. 5,348,832; 5,405,728; 5,366,841; 5,496,676; 5,527,658; 5,585,215; 5,650,255; 5,650,256; 5,501,935; 5,723,253; 5,744,520; 5,763,133; 5,766,818; 5,747,215; 5,827,633; 5,853,944; 5,804,349; 5,840,462; 5,869,215; 5,863,698; 5,902,710; 5,910,387; 5,916,725; 5,919,595; 5,925,488, and 5,977,210. The appropriate components and processes of the above Xerox Corporation patents can be selected for the processes of the present invention in embodiments thereof. In both the above described conventional process and in processes such as the EA process, surface additive particles are added using high intensity blending processes.
High speed blending of dry, dispersed, or slurried particles is a common operation in the preparation of many industrial products. Examples of products commonly made using such high-speed blending operations include, without limitation, paint and colorant dispersions, pigments, varnishes, inks, pharmaceuticals, cosmetics, adhesives, food, food colorants, flavorings, beverages, rubber, and many plastic products. In some industrial operations, the impacts created during such high-speed blending are used both to uniformly mix the blend media and, additionally, to cause attachment of additive chemicals to the surface of particles (including resin molecules or conglomerates of resins and particles) in order to impart additional chemical, mechanical, and/or electrostatic properties. Such attachment between particles is typically caused by both mechanical impaction and electrostatic bonding between additives and particles as a result of the extreme pressures created by particle/additive impacts within the blender device. Among the products wherein attachments between particles and/or resins and additive particles are important during at least one stage of manufacture are paint dispersions, inks, pigments, rubber, and certain plastics.
High intensity blending typically occurs in a blending machine, and the blending intensity is greatly influenced by the shape and speed of the blending tool used in the blending process. A typical blending machine and blending tool of the prior art is exemplified in FIGS. 1 and 2. FIG. 1 is a schematic elevational view of a blending machine 2. Blending machine 2 comprises a vessel 10 into which materials to be mixed and blended are added before or during the blending process. Housing base 12 supports the weight of vessel 10 and its contents. Motor 13 is located within housing base 12 such that its drive shaft 14 extends vertically through an aperture in housing 12. Shaft 14 also extends into vessel 10 through sealed aperture 15 located at the bottom of vessel 10. Upon rotation, shaft 14 has an axis of rotation that generally is orthogonal to the bottom of vessel 10. Shaft 14 is fitted with a locking fixture 17 at its end, and blending tool 16 is rigidly attached to shaft 14 by locking fixture 17. Before blending is commenced, lid 18 is lowered and fastened onto vessel 10 to prevent spillage. For high intensity blending, the speed of the rotating tool at its outside edge generally exceeds 50 ft./second. The higher the speed, the more intense, and tool speeds in excess of 90 ft./second, or 120 ft./second are common.
Various shapes and thicknesses of blending tools are possible. Various configurations are shown in the brochures and catalogues offered by manufacturer's of high-speed blending equipment such as Henschel, Littleford Day Inc., and other vendors. The tool shown in FIG. 1 is based upon a tool for high intensity blending produced by Littleford Day, Inc. and is discussed in more detail in relation to FIG. 3 discussed below. Among the reasons for different configurations of blending tools are (i) different viscosities often require differently shaped tools to efficiently utilize the power and torque of the blending motor; and (ii) different blending applications require different intensities of blending. For instance, some food processing applications may require a very fine distribution of small solid particles such as colorants and flavorings within a liquid medium. As another example, the processing of snow cones requires rapid and very high intensity blending designed to shatter ice cubes into small particles which are then mixed within the blender with flavored syrups to form a slurry.
As discussed more fully below, the shape of blending tool 16 greatly affects the intensity of blending. One type of tool design attempts to achieve high intensity blending by enlarging collision surfaces, thereby increasing the number of collisions per unit of time, or intensity. One problem with this type of tool is that particles tend to become stuck to the front part of the tool, thereby decreasing efficiency and rendering some particles un-mixed. An example of an improved tool using an enlarged collision surface that attempts to overcome this “snow-plowing” effect is disclosed in U.S. Pat. No. 6,523,996 entitled “BLENDING TOOL WITH AN ENLARGED COLLISION SURFACE FOR INCREASED BLEND INTENSITY AND METHOD OF BLENDING TONERS,” hereby incorporated by reference. Even when overcoming the “snow-plow” effect, a second limitation of prior art tools with enlarged collision surfaces is that particles in the blender tend to swirl in the direction and nearly at the speed of the moving tool. Thus, the impact speed between the tool and a statistical average of particles moving within vessel 10 is less than the speed of the tool itself since the particles generally are moving the same direction as the tool.
Another type of a blending tool that is more typically used for blending toners and additives is shown in FIG. 2 as tool 26. As shown, tool 26 comprises 3 wing shaped blades, each arranged orthoganally to the blade immediately above and/or below it. Tool 26 as shown has blades 27, 28, and 29. Blade 27, the bottom blade, is generally called “the scraper” and serves to lift particles from the bottom and provide initial motion to the particles. Blade 28, the middle blade, is called “the fluidizing tool” and serves to provide additional mechanical energy to the mixture. Blade 29, the top blade, is called the “horn tool” and is usually bent upward at an angle. The high speed distal tips proximal the wall of the blending vessel are primarily responsible for additive dispersion and inducing/providing impact/shear energy to attach the additive particles to the toner. Since tool 26 is designed such that each of its separate blades are relatively thin and therefore flow through the toner and additive mixture without accretion of particles on the leading edges, measure of the power consumed by the blending motor is a good indicator of the intensity of blending that occurs during use of the tool. This power consumption is measured as the specific power of a tool, defined as follows:
      Specific    ⁢                  ⁢    Power    =                              Load          ⁢                                          ⁢          Power                -                  No          ⁢                                          ⁢          Load          ⁢                                          ⁢          Power                            Batch        ⁢                                  ⁢        Weight              ⁡          [              Watt        ⁢                  /                ⁢                  lb          .                    ]      The Specific Power of tool 26 is shown in FIG. 8 in relation to different speeds of rotation. The significance of the data shown in FIGS. 9 and 10 is discussed below when describing advantages of an embodiment of the present invention. It should be noted, however, that tool 26 also embodies the limitation described above wherein the actual collision energy between particles is usually less than the speed of the tool itself since each of blades 27, 28, an 29 have the effect of swirling particles within the blending vessel in the direction of tool rotation.
Some tools of the prior art are designed to achieve blend intensity through creation of vortices and shear forces. One such tool is sold by Littleford Day Inc. for use in its blenders and appears in cross-section as tool 16 in FIG. 1. As shown in perspective view in FIG. 3, the Littleford tool 16 has center shank 20 with a central bushing fixture 17A for engagement with locking fixture 17 at the end of shaft 14 (both fixture 17 and shaft 14 are shown in FIG. 1). Bushing fixture 17A includes a notch conforming to a male locking key feature on locking fixture 17 (from FIG. 1). Arrow 21 shows the direction in which tool 16 rotates upon shaft 14. A second scraper blade 16A may be mounted below tool 16 onto shaft 14 as shown in FIG. 3. In the configuration shown, the Littleford scraper blade 16A comprises a shank mounted orthogonally to center shank 20 that emerges from underneath shank 20 in an essentially horizontal manner and then dips downward near its end region. The end region of blade 16A is shaped into a flat club shape with a leading edge near the bottom of the blending vessel (not shown) and the trailing edge sloping slightly upward to impart lift to particles scraped from the bottom of the vessel. The leading edge of the club shape runs from an outside corner nearest the blending vessel wall inwardly towards the general direction of shaft 14. The scraper blades are shorter than shank 20, and the combination of this shorter length plus the shape of the leading edge indicates that the function of the Littleford scraper blade is to lift particles in the middle of the blending vessel upward from the bottom of the vessel.
In contrast to the tool shown in FIG. 2, tool 16 comprises vertical risers 19A and 19B that are fixed to the end of center shank 20 at its point of greatest velocity during rotation around central bushing 17A. These vertical risers 19A and 19B are angled, or canted, in relation to the axis of center shank 20 at an angle of 17 degrees. In this manner, the leading edges 21A and 21B of risers 19A and 19B are proximate the wall of blending vessel 10 (from FIG. 1) while the trailing edges 22A and 22B are further removed from vessel wall 10. Applicant believes that tool 16 operates by creating shear forces between particles caught in the space created between the outside surface of risers 19A and 19B and the wall of vessel 10. Since trailing edges 22B and 22A are further removed from the wall, a vortex is created in this space. It is believed that particles trapped in these vortices follow the tool at or nearly at the speed of leading edges 19A and 19B. In contrast, particles that have slipped through gap between leading edge 19A and 19B and the wall of vessel 10 remain nearly stationary. When particles swept along within the vortices behind leading edges 19A and 19B impact the nearly stationary particles along the vessel wall, then the speed of collision is at or nearly at the speed of the leading edges of the tool. Applicant has not found literature that describes the above effects. Instead, the above analysis results from Applicants' own investigation of blending tools.
An improvement upon the Littleford tool shown in FIGS. 1 and 3 is disclosed in U.S. Pat. No. 6,752,561, issued Jun. 22, 2004 to Kumar et al, which is hereby incorporated herein in its entirety. The tool of the '561 patent is shown in FIG. 4 and comprises a shank having a riser member at each end, such risers being angled to the axis of the shank between 10 and 16 degrees and having a height dimension greater that 20 percent of the diagonal dimension of the shank.
Although the tool shown in FIG. 4 has proven to achieve the specific power and blend intensity described in the '561, several problems have arisen. Specifically, experience has shown that the tool in FIG. 4 is prone to a static toner accumulation proximate to the rear of the inside edge of each riser. Toner that aggregates in such accumulation does not get blended adequately. A certain amount of such inadequately blended toner typically becomes loose during or after the blending operation, thereby resulting in a portion of a toner batch having inadequate additive coverage or adhesion. Without sufficient additive adhesion and coverage, the affected toner particles are likely to perform poorly during imaging operations. Accordingly, it would be very desirable to design a blending tool that creates substantially the same specific power and blending intensity as the tool described in the '561 patent but which incurs little or no static toner accumulation.
A second problem with the tool disclosed in the '561 patent is that the intense centrifugal forces imposed on the tool tends to bend the shank downward and, separately, the risers outward. Together, these deflections can cause structural failure of the tool. The bending is sufficient to permanently deform the risers outward from the intended vertical angle to the shank. Even without structural failure of the tool, such deflections can cause the tool to touch the blend chamber wall at high rotation speeds. The root cause of the deflections is the extreme bending moments of the tool at high rotation speeds that cause local stress levels to exceed the yield stress of the material. Although the tool can be reinforced with more material to inhibit deflection, such reinforcement increases tool mass, thereby decreasing blending efficiency while modestly increasing the amount of toner accumulation on the riser inside edge.
A third problem resulting from use of the tool of the '561 patent is that temperatures within the blending vessel may become undesirably high. When blending toners with the '561 tool, temperatures of 130 F are common. Such temperatures are uncomfortably close to the transition temperature of toner resins and, accordingly, risk melting and fusing of toner particles within the blending vessel.
As described above, the process of blending plays an increasingly important role in the manufacture of electrophotographic and similar toners. It would be advantageous if a blending tool design and blending method were found that achieves at least the same specific power and blending intensity as the tool of the '561 patent while minimizing static powder accumulation and outward deflection of the tool risers while maintaining temperatures within the blending vessel well below transition temperatures of typical toner resins.